Enzyme histochemistry in muscle and nerve biopsies is carried out for diagnostic purposes in patients with several neuromuscular disorders. Immunocytochemical studies were conducted using specific antibodies to thymic peptides to investigate changes in the distribution of epithelial cells in the thymus of patients with myasthenia gravis. The interaction between cells of the lymphoid and central nervous system was investigated searching for common antigenic markers on their cell surface. Thymosin beta an immunomodulating polypeptide, was found to b a common antigen shared by macropahges, dendritic lymphoid cells and oligodendrocytes; in contrast Thymosin-a was found present in astrocytes. The IgM of certain patients with paraproteinemic polyneuropathies has been identified as a specific antibody to myelin associated glycoprotein or glycolipids; nerve biopsies from these patients are studied by electron microscopy and immunocytochemically. The nature of amyloid protein in patients with "sporadic" amyloid polyneuropathy was identified using specific antibodies to amyloid proteins immunocytochemically and biochemically on the extracted amyloid. The mechanism of inflammatory myopathy in monkeys with immunodeficiency (Simian AIDS) due to a retrovirus D. (SRV-1), was further studied. Antibodies to SRV-1 immunoreacted with the inflammatory cells invading the muscle fibers; SRV-1 was capable of infecting myoblasts in tissue culture without exerting a cytopathic effect in the muscle. The mechanism of polymyositis and inflammatory neuropathies in patients with AIDS is being investigated wit in situ hybridization and immunocytochemistry. The effect of aging on the neuromuscular system of monkeys from age 5 to 25 is being investigated with a detailed morphological, electrophysiological and morphometrical analysis of their muscle and nerve biopsies. Morphological changes in the muscles of autoimmune mice are also studied; tubular aggregates were found and their relation to endogenous interferon is being investigated.